Twin waveguide based design for photonic integrated circuits

ABSTRACT

An asymmetric twin waveguide (ATG) structure is disclosed that significantly reduces the negative effects of inter-modal interference in symmetric twin-waveguide structures and which can be effectively used to implement a variety of optical devices. The ATG structure of the invention can be monolithically fabricated on a single epitaxial structure without the necessity of epitaxial re-growth. To achieve the ATG structure of the invention, the effective index of the passive waveguide in the ATG is varied from that of a symmetric twin waveguide such that one mode of the even and odd modes of propagation is primarily confined to the passive waveguide and the other to the active waveguide. The different effective indices of the two coupled waveguides result in the even and odd modes becoming highly asymmetric. As a result, the mode with the larger confinement factor in the active waveguide experiences higher gain and becomes dominant. In a further embodiment, the active waveguide is tapered to reduce coupling losses of the optical energy between the passive waveguide and the active waveguide. In a further embodiment, a grating region is incorporated atop the passive waveguide to select certain frequencies for transmission of light through the passive waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/642,316 filed Aug. 15, 2003 now U.S. Pat. No. 6,819,814 and entitled“Twin Waveguide Based Design for Photonic Integrated Circuits,” which isa continuation of U.S. patent application Ser. No. 09/982,001 filed Oct.18, 2001 and entitled “Twin Waveguide Based Design for PhotonicIntegrated Circuits,” which is a continuation of U.S. patent applicationSer. No. 09/337,785 filed Jun. 22, 1999 and entitled “Twin WaveguideBased Design for Photonic Integrated Circuits,” now U.S. Pat. No.6,381,380, which claims priority to U.S. Provisional Application60/090,451 filed Jun. 24, 1998 and entitled “Twin Waveguide Based Designfor Photonic Integrated Circuits,” the contents of all of which arehereby incorporated by reference in their entirety.

GOVERNMENT LICENSE RIGHTS

The U.S. Government may have a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Grant NumberF19628-94-C-0045 awarded by DARPA.

FIELD OF THE INVENTION

The present invention is related to the field of optical communications,and more particularly to waveguide design in photonic integratedcircuits.

BACKGROUND OF THE INVENTION

Photonic integrated circuits (PIC) provide an integrated technologyplatform increasingly used to form complex optical circuits. The PICtechnology allows many optical devices both active and passive, to beintegrated on a single substrate. For example, PICs may compriseintegrated lasers, integrated receivers, waveguides, detectors,semiconductor optical amplifiers (SOA), and other active and passiveoptical devices. Such monolithic integration of active and passivedevices in PICs provides an effective integrated technology platform foruse in optical communication.

A particularly versatile PIC platform technology is the integrated twinwaveguide (TG) structure in which active and passive waveguides arecombined in a vertical directional coupler geometry using evanescentfield coupling. As is known, the TG structure requires only a singleepitaxial growth step to produce a structure on which active and passivedevices are layered and fabricated. That is, TG provides a platformtechnology by which a variety of PICs, each with different layouts andcomponents, can be fabricated from the same base wafer. All of theintegrated components are defined by post-growth patterning, eliminatingthe need for epitaxial regrowth. Additionally, the active and passivecomponents in a TG-based PIC can be separately optimized withpost-growth processing steps used to determine the location and type ofdevices on the PIC.

The conventional TG structure, however, suffers from the disadvantagethat waveguide coupling is strongly dependent on device length, due tointeraction between optical modes. A common problem in prior-art TGstructure: is the relative inability to control the lasing thresholdcurrent and coupling to the passive waveguide as a consequence of thesensitivity to variations in the device structure itself. Thesensitivity variations arise form the interaction between the even andthe odd modes of propagation in the conventional TG structure. Thisinteraction leads to constructive and destructive interference in thelaser cavity, which affects the threshold current, modal gain, couplingefficiency and output coupling parameters of the device. It is notedthat the threshold current represents the value above which the laserwill lase, the modal gain is the gain achieved by traveling through themedium between the laser facets, and the coupling efficiency is thepercentage of optical power transference between the active and passiveregions in the optical device. In sum, the conventional TG structuresuffers from unstable sensitivity in performance characteristics due tolaser cavity length, even/odd mode interaction and variations in thelayered structure.

A modified TG structure was disclosed in U.S. Pat. No. 5,859,866 toForrest et al., which addressed some of the performance problems of theconventional TG structure by adding an absorption layer (or loss layer)between the upper and lower waveguides, thereby introducing additionalloss to the even mode so that its interaction with the odd mode isattenuated. That patent, which includes common inventors with theinvention described herein, is hereby incorporated by reference herein.The modified TG structure described in the '866 patent is designed tohave relatively equal confinement factors for both the even and oddmodes in each waveguide layer by constructing active and passivewaveguides of equal effective indices of refraction. The resultingconfinement factors are relatively the same because the even and oddoptical modes are split relatively equally in the active and passivewaveguides. The absorption layer in the modified TG structure suppresseslasing on the even mode, thereby making the TG coupling efficiencyindependent of laser cavity length. The absorption layer substantiallyeliminates the propagation of the even mode, while having minimal effecton the odd mode. With the substantial elimination of even-modepropagation by the absorptive layer, modal interaction is largelyeliminated, resulting in optical power transfer without affectingperformance parameter such as the threshold current, modal gain,coupling efficiency and output coupling.

However, the modified TG structure of the '866 patent is ineffective ina device with a traveling-wave optical amplifier (TWA), which is animportant component in PICs designed for optical communication systems.In a TG device with an absorption layer operated as a TWA, theadditional absorption in the single pass through the active region isinsufficient to remove the even mode. It is desirable to have a commonoptical structure that can be effectively utilized for integrating bothlasers and TWAs.

Therefore, there is a need in the art of optical communications toprovide a relatively simple and cost-effective integration scheme foruse with a traveling-wave optical amplifier (TWA).

There is a further need in the art to provide a twin waveguide (TG)structure that ensure stability in the laser and the traveling-waveoptical amplifier (TWA).

There is a further need in the art to provide a TG structure thatsignificantly reduces negative effects of modal interference without theconcomitant coupling loss.

There is a further need in the art to provide a TG structure with theaforementioned that can be monolithically fabricated on a singleepitaxial structure.

SUMMARY OF THE INVENTION

The invention provides an asymmetric twin waveguide (ATG) structure thatsignificantly reduces the negative effects of modal interference andwhich can be effectively used to implement both lass and traveling-waveoptical amplifiers (TWA). The ATG in the invention advantageouslyensures stability in the laser and the TWA. In addition, the ATGprovided in the invention can be monolithically fabricated on a singleepitaxial structure without the necessity of epitaxial re-growth. Mostimportantly, the ATG, according to the present invention, is a versatileplatform technology by which a variety of PICs, each with differentlayouts and components, can be fabricated from the same base wafer andmodified with conventional semiconductor processing techniques toproduce substantial modal gains and negligible coupling losses betweenPIC components.

In an embodiment of the ATG structure of the invention, the effectiveindex of one of the passive waveguides in the ATG is varied from that ofa symmetric twin waveguide such that one mode of the even and odd modesof propagation is primarily confined to the passive waveguide and theother to the active waveguide. As a result, the mode with the largerconfinement factor in the active waveguide experiences higher gain andbecomes dominant.

In an illustrative embodiment, monolithic integration of a 1.55 μmwavelength InGaAsP/InP multiple quantum well (MQW) laser and atraveling-wave optical amplifier (TWA) is achieved using the ATGstructure of the invention. The laser and the amplifier shin the samestrained InGaAsP MQW active layer grown by gas-source molecular beamepitaxy, while the underlying passive waveguide layer is used foron-chip optical interconnections between the active devices. In thisparticular embodiment, the passive waveguide has a higher effectiveindex than the active waveguide, resulting in the even and odd modesbecoming highly asymmetric. An appropriate combination of the thicknessand index of refraction of the materials chosen for the waveguidesresults in modifying the effective index of refraction. The ATGstructure uses the difference in modal gains to discriminate between theeven and odd modes.

In a further embodiment, the active waveguide in a monolithicallyintegrated device is laterally tapered by conventional semiconductoretching techniques. The tapered region of the active waveguide, at ajunction of active and passive devices, helps to reduce coupling lossesby resonant or adiabatic coupling of the optical energy between thepassive waveguide and the active waveguide. As a result, the modal gainis significant compared to the symmetric TG structure and the couplingloss in the non-tapered ATG structure is reduced to negligible levels.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be obtainedby considering the following description in conjunction with thedrawings in which:

FIG. 1 is a refractive index profile of the even and the odd modes ofthe asymmetric twin waveguide (ATG) structure in accordance with thepresent invention.

FIG. 2 is a schematic view of the ATG structure in accordance with thepresent

FIG. 3 shows a schematic view illustrative of device fabrication for theATG structure of the present invention.

FIG. 4 is a three-dimensional schematic of the ATG structure including ataper coupler in accordance with the present invention.

DETAILED DESCRIPTION

As already noted in the Background, the twin-waveguide approach tophotonic integration represents a versatile platform technology by whicha variety of PICs, each with different layouts and components, can befabricated from the same base wafer—that wafer being grown in a singleepitaxial growth step. Typically, the upper layer is used for activedevices with gain (e.g., lasers SOAs), whereas the lower layer, with alarger bandgap energy, is used for on-chip manipulation of the opticenergy generated by the active device(s) via etched waveguides. Withsuch a TG structured PIC, active components such as semiconductoroptical amplifiers (SOAs), Fabry-Perot and single frequency distributedBragg reflector (DBR) lasers can be integrated with passive componentssuch as Y-branches and multi-beam splitters, directional couplers,distributed Bragg feedback grating sections, multimode interference(MMI) couplers and Mach-Zehnder modulators.

As previously noted, the simple TG structured PIC suffers from a strongdependence between waveguide coupling and device length, due to theinteraction between optical modes. For TG lasers, this problem has beenaddressed by the addition of an absorption layer between the upper andlower waveguides, as disclosed in cross-referenced U.S. Pat. No.5,859,866. Such an inserted absorption layer introduces additional lossto the even mode, thereby attenuating its interaction with the odd mode.However, the loss layer concept cannot be effectively applied to asingle-pass or traveling-wave optical amplifier (TWA), where both theeven and odd modes must be considered. In a TG structure incorporating aTWA, the additional absorption in the single pass through the activeregion is insufficient to remove the even mode, since in a TWA,reflectivity is suppressed for both facets of the semiconductor laser.

Accordingly, a new, more advantageous approach to mode selection in a TGis disclosed herein—an asymmetric twin waveguide structure, which can beeffectively utilized with a TWA and a laser. With a symmetric TG, asdescribed above, equal confinement factors exist for both the even andodd modes in each waveguiding layer. This permits nearly complete powertransfer between the guides and the maximum output coupling at an etchedhalf-facet is 50 percent for either mode. With the asymmetric twinwaveguide (ATG) structure of the invention, on the other hand, theeffective index of the passive or active waveguide layer is changedrelative to that used in a symmetric TG structure. As a result ofdiffering effective indices of refraction the even and odd modes ofpropagation are split unequally between the waveguides. The unequalsplitting is shown graphically in FIG. 1, which illustrates the modalintensity and refractive index profile of the ATG structure of theinvention. As will be seen in the figure, in this particular case, theodd mode is primarily confined to the active waveguide, while the evenmode is more strongly confined to the passive waveguide. The figure alsoshows, for an illusive embodiment of the invention described below, thecalculated confinement factors for both modes in the quantum wells(Γ^(QW)) in the active waveguide, and their coupling coefficients to thepassive waveguide (C_(o), C_(e) for odd and even modes, respectively).

With the ATG structure of the invention, the odd mode has higher gainand reflectivity at the etched facet, and therefore easily dominates inan ATG laser. Accordingly, for such an ATG laser, the absorption layerneeded for the symmetric TG is not warranted. However, for a travelingwave optical amplifier (TWA) implemented in the ATG active waveguide,the situation is more complex, because both modes must be considered. Aslight enters the ATG TWA section, it splits between the even (e) and odd(o) modes with the amplitude coupling coefficients, C_(o) and C_(e)equal to the overlap integrals of the corresponding modes with the modeof the passive guide. The same coupling coefficients apply at the end ofthe TG section. Ignoring gain saturation effects, the totalinput-to-output electric-field transmission ratio is:E _(out) /E _(in) =C _(e) ²exp(Γ_(e) ^(QW) gL/2)+C _(o) ²exp(Γ_(o) ^(QW)gL/2)exp(iΔk·L)where g is the gain of the quantum well stack, L is the length of the TGsection, and Δk·L is the phase difference between the even and odd modesat the amplifier output due to their slightly different propagationconstants. For sufficiently large gL, the odd mode is amplified muchmore than the even, and dominate the TWA output regardless of phase. Inthis circumstance, the even mode can be ignored, and the input-to-outputpower gain isP _(out) /P _(in) =C _(o) ⁴exp(Γ_(o) ^(QW) gL).Hence, the ATG structure of the invention uses gain, rather than a losslayer, to discriminate between the modes. This ensures stability of bothATG lasers and TWAs by reducing mode interference effects.

An illustrative embodiment of the invention is depicted schematically inFIG. 2. In the illustrated ATG structure 11, shown in verticalcross-section in the figure, two stacked waveguide layers 61 and 71 areseparated by cladding layers 31 and 41. The active waveguide 71incorporates multiple quantum wells 115 for high gain. For an exemplaryembodiment, six such quantum wells are selected, and the activewaveguide implements a laser and a TWA. Vertical facets 150 and 160 areformed in the active waveguide for the laser and the TWA. Passive region61 incorporates a passive waveguide 125 for propagating light emittedfrom the active waveguide. The refractive indices and thickness of thewaveguide layers are chosen to achieve a 30:70 ratio of confinementfactors in the passive guide for the odd and even modes, respectively.The resulting quantum well confinement factors are 11% for the odd and5% for the even mode.

Fabrication of this illustrative ATG structure, which is depictedschematically in FIG. 3, is carried out using gas-source molecular beamepitaxy on an S-doped (100) n+ InP substrate. After epitaxial growth,active regions of the laser and TWA are masked using a 3000 Å thicklayer of plasma-deposited SiN_(χ). The unmasked areas are etched to thebottom of the first waveguide using reactive ion etching in a CH₄:7H₂plasma at 0.8 W/cm². This etch removes the upper waveguide layer andquantum wells from the passive regions of the device, and at the sametime, forms the vertical facets (150 and 160 of FIG. 2) for the laserand TWA.

A second, 5 μm-wide SiN_(χ) mask is then used to define the ridgewaveguide. This ridge (as shown in FIG. 3) runs perpendicular to theetched facet in the laser section, and is tilted at a 7° angle from thenormal position at both TWA facets in order to prevent optical feedbackinto the amplifier. The ridge waveguide is formed by material-selectivewet etching using a 1H₂SO₄:1H₂O₂:10H₂O for InGaAsP, and 3HCl:1H₃PO₄ forInP. The ridge is about 3.8 μm wide, and supports a single lateral mode.The ridge height in the active and passive regions is different,controlled by two InGaAsP etch-stop layers. During the wet etchingprocess, the dry-etched facets of the laser and TWA are protected by theridge mask which is continuous on the vertical walls. Followingdeposition of the isolation SiN_(χ), the wafer is spin-coated withphotoresist which is then etched in an O₂ plasma until the top of theridge is exposed. The SiN_(χ) is then removed from the ridge, followedby the removal of the photoresist. In the next step, the p- andn-contacts are electron-beam deposited using Ti/Ni/Au (200/500/1200 Å)and Ge/Au/Ni/Au (270/450/215/1200 Å), respectively. Finally, the rearlaser facet and the TWA output waveguide are cleaved.

With the ATG structure of the invention as heretofore described, theconfinement factors for the two optical modes (odd and even) are splitunequally between the active and passive waveguides. As a result, one ofthe modes is primarily confined to the passive waveguide and the otherto the active waveguide. The mode which is contained primarily in theupper waveguide experiences higher gain and becomes dominant. Thus, theATG structure provides a gain advantage, and generally higher stability,over a symmetric TG structure. However, the ATG structure also producesa relatively larger coupling loss than is experienced with the symmetricTG. While the higher gain for the ATG structure more than offsets thisrelative disadvantage in coupling loss, it would be desirable to providean ATG structure with lower coupling loss. To that end, a furtherembodiment of the invention is disclosed herein which improves theefficiency of coupling power between the active to the passive waveguideand back in an ATG.

In particular, this further embodiment of the invention applies alateral taper on the active waveguide to induce coupling between theactive region and the adjacent passive region. This implementationdrastically reduces coupling losses between the waveguide layers whileretaining the absolute gain for the dominant mode in the active region.The performance of such an ATG combined with a taper on the activewaveguide rivals the performance of devices previously possible onlyusing complicated epitaxial regrowth processes.

Referring to FIG. 4, there is shown an exemplary embodiment of an ATGtaper coupler in accordance with the invention. The exemplary ATGstructure 11 of FIG. 4 incorporate a 2.4 μm wide shallow ridge waveguidein the upper active layer having an effective index higher than that ofthe lower passive layer. Hence, the even mode of propagation has a highconfinement factor in the multiple quantum well active region. Underthis condition, only the even mode of a Fabry-Perot laser will undergosignificant gain. The coupling of this amplified mode into the passivelayer at the end of the gain region is accomplished by increasing theetch depth of the waveguide ridge through the active layer to form ahigh-contrast lateral waveguide followed by a lateral taper region 81.For the exemplary embodiment, an exponential taper is used, which has asmaller mode transformation loss than a linear taper. It should,however, be understood that tapers of other shapes, as well asmulti-section tapers, may be incorporated into the active waveguide andare within the contemplation of the invention.

At a tapered waveguide width of 1.1 μm for the exemplary embodiment, theeffective indices of the two guides are matched and the power couplesinto the lower waveguide. As the taper narrows further, its effectiveindex becomes smaller than that of the passive guide, in effect, lockingthe mode into the lower layer. This coupling arrangement is largelyinsensitive to small wavelength changes as long as the untapered ATGstructure remains strongly asymmetric.

Fabrication of the exemplary ATG taper coupler is as follows: An InGaAsPpassive waveguide 61 is first grown on a n+ doped (100) InP substrate51. The passive waveguide 61 is 0.5 μm thick and has an energy gapcutoff wavelength of λ_(g) of 1.2 μm. An InP cladding layer 41 ofthickness 0.5 μm is followed by an InGaAsP active waveguide 71 with anenergy gap cutoff wavelength of λ_(g) of 1.20 μm. The active waveguide71 incorporates six 135 Å thick, 1% compressively strained InGaAsPquantum wells separated by 228 Å barriers. An InP top cladding layer 31is grown to a thickness of 1.2 μm and then a p+ InGaAsP contact layer 21of 0.2 μm thickness is grown on top of the top cladding layer 31.

Once the basic twin-guide structure has been grown, a laser ridgewaveguide with tapers at both ends is etched in a CH₄/H₂ (1:7) plasma at0.8 W/cm² using a SiN_(X) mask. The 1.2 μm high ridge terminatesapproximately 0.2 μm above the active waveguide. Next, a second, wideSiN_(X) mask is added to cover the laser gain region but not the tapers.Etching is continued through the active waveguide defining the verticalwalls of the taper and the etched facet, the latter being tilted at anangle of 7° from the waveguide longitudinal axis to prevent unwantedreflections. Next, the 700 nm high passive ridge is patterned andetched, extending 0.2 μm into the lower waveguide. After etching, a 3000Å thick SiN_(X) electrical isolation layer is deposited, followed by aTi/Ni/Au (200/500/1200 Å) p-contact patterned using a self-alignedphotoresist process. Finally, the wafer is thinned to approximately 100μm and the Ge/Au/Ni/Au (270/450/215/1200 Å) n-contact is deposited andannealed at 360° C.

The inventors have empirically concluded that additional loss in theintegrated devices due to the taper couplers is negligible. Empiricalresults also show that an ATG taper coupler with integrated lasers withL_(A)=2.05 mm produced output powers ≦ approximately 35 mW with 24%slope efficiency per facet. Imaging the facets with an infrared videocamera clearly shows that almost all of the power is emitted from thewaveguide, with very little light scattered from the tapered region.

In a further embodiment, a grating region is incorporated atop thepassive waveguide. The grating region can be conventionally etched orformed on the passive waveguide and can be shaped with triangular peakor can be sinusoidal or rectangular in shape with repeating patterns.The grating region is used to select certain frequencies fortransmission of light through the passive waveguide. By selectivelyadjusting the period of the grating region, the frequency to bereflected can be selected.

The invention can also be embodied in other integrated devices, usinglasers and TWAs as the active components, interconnected by waveguidesformed in passive layers using tapers at each active-to-passive junctionproviding low-loss optical coupling of light between adjacent sections.

CONCLUSION

A monolithically integrated InGaAsP/InP MQW laser and optical amplifierare disclosed herein, using a novel, asymmetric twin-waveguide (ATG)structure which uses gain to select one of the two propagating modes.The ATG structure can be effectively utilized with a traveling-waveamplifier (TWA), where performance up to 17 dB internal gain and lowgain ripple can be obtained.

The ATG structure differs from the prior art symmetric twin waveguidestructure in that the two optical modes are split unequally between theactive and passive waveguides. This is achieved by varying the effectiveindex of the waveguides slightly from that required by the symmetricmode condition. As a result, one of the modes is primarily confined tothe passive waveguide. The mode with the larger confinement factor inthe active waveguide experiences higher gain and becomes dominant. Asmaller coupling ratio for the dominant mode compared to that in thesymmetric structure is offset by higher gain for that mode due to itsconfinement factor of the active region therein which is larger thanthat of the symmetric TG.

The ATG structure of the invention uses a single material growth step,followed by dry and wet etching steps to delineate the active andpassive devices in the upper and lower waveguides of the TG structure.

In a further embodiment, the ATG structure of the invention isintegrated with a taper coupler to retain the higher gain possible withan ATG while reducing the coupling losses between the active and passivedevices made from the ATG structure.

Although the present invention is described in various illustrativeembodiments, it is not intended to limit the invention to the preciseembodiments disclosed herein. Accordingly, this description is to beconstrued as illustrative only. Those who are skilled in this technologycan make various alterations and modifications without departing fromthe scope and spirit of this invention. Therefore, the scope of thepresent invention shall be defined and protected by the following claimsand their equivalents. The exclusive use of all modifications within thescope of the claims is reserved.

1. A monolithically integrated optical circuit comprising: an activeregion for emitting light, said active region formed in a first layer ofthe optical circuit; and a passive region for propagating said light,said passive region coupled to said active region and formed in a secondlayer of the optical circuit; wherein said active region has a firsteffective index of refraction and said passive region has a secondeffective index of refraction, said first effective index of refractionand said second effective index of refraction having values causing afirst mode of light to propagate primarily in said active region and asecond mode of light to propagate primarily in said passive region, andwherein a first optical device is formed in said active region, a secondoptical device is formed in said active region, and said passive regioncommunicates light between said first optical device and said secondoptical device.
 2. An integrated optical circuit according to claim 1,further comprising a layer positioned between said active region andsaid passive region, said layer having a lower index of refraction thansaid active region and said passive region, and allowing for coupling oflight between said active region and said passive region.
 3. Anintegrated optical circuit according to claim 1, wherein said firsteffective index of refraction has a value causing the first mode to beprimarily confined to said active region and said second effective indexof refraction has a value causing the second mode to be primarilyconfined in said passive region.
 4. An integrated optical circuitaccording to claim 1, wherein said first effective index of refractionand the second effective index of refraction have values causingapproximately 70% or more of one of said first and second modes to beconfined to the active region.
 5. An integrated optical circuitaccording to claim 1, wherein said first optical device comprises afirst waveguide formed in said active region, said second optical devicehas a second waveguide formed in said active region, and said passiveregion has a third waveguide formed therein for guiding light betweenthe between said first and second waveguides.
 6. An integrated opticalcircuit according to claim 5, wherein said first waveguide comprises alateral taper at its junction with the third waveguide, and said secondwaveguide includes a lateral taper formed at its junction with the thirdwaveguide.
 7. An integrated optical circuit according to claim 6,wherein said lateral tapers follow exponential curves.
 8. An integratedoptical circuit according to claim 5, wherein said first waveguide andsaid second waveguide are shallow ridge waveguides and have effectiveindices of refraction higher than the third waveguide.
 9. An integratedoptical circuit according to claim 1, wherein said first optical deviceis a laser.
 10. An integrated optical circuit according to claim 9,wherein said laser is driven by at least one quantum well.
 11. Anintegrated optical circuit according to claim 9, wherein said secondoptical device is a semiconductor optical amplifier.
 12. An integratedoptical circuit according to claim 11, wherein said semiconductoroptical amplifier is a travelling-wave optical amplifier.
 13. Anintegrated optical circuit according to claim 1, wherein said passiveregion comprises a grating region for reflecting selected frequencies oflight from said active region.
 14. An integrated optical circuitaccording to claim 1, wherein said passive region comprises a thirdoptical device.
 15. An integrated optical device according to claim 14,wherein said third optical device operates on light communicatingbetween said first optical device and said third optical device.